In the photos:

• Orange cat is without this adapter
• Black cat is with the adapter

This is a 25mm to 50mm diameter adapter. I noticed a very substantial improvement in print quality after installing it.

It improves how easily the Extruder Motor can pull the filament out of the spool by decreasing the gap between the holder and the spool, this avoids "jumps" in filament tension and gives a more uniform flow at the nozzle.

To further improve the filament pull uniformity, I also suggest mounting the spool holder to the side of your printer, this will effectively reduce the max print height to about 18cm, but the improvement in print quality is well worth it!

Note: This adapter does not have it's own stopper, the threaded one that comes with your ender-3 stops this adapter, but not the spool; this means the spool may fall of the holder, happened just once with me. my suggestions are:

• Cardboard cutout with a diameter of 25mm on the inside and 100mm on the outside,
• Print you own Stopper by resizing this model and gluing it together with the adapter:

X and Y Axis: 135%
Z Axis: 5%

• Tape a pencil to the holder (like I did)
  In all cases, use the threaded stopper provided with your ender-3